The present invention concerns suspended load lifting apparatus, and more particularly tower cranes. This invention is even more particularly relevant to a motorized height access device for a tower crane enabling:                on the one hand, the crane driver to reach quickly, without effort or fatigue, a driving cab situated in the high part of the tower crane;        on the other hand, operators to access various heights during operations of mounting or dismantling the structure of the tower crane or for other work.        
A tower crane conventionally comprises a vertical mast at the top of which is mounted a boom turning about a vertical axis. Along the boom moves a carriage, below which the load to be lifted is suspended from a hook. The driving cab of such a tower crane is usually formed of a cabin situated in the region at or near the top of the mast and thus at a great height above the ground.
In most tower cranes access to the driving cab for the crane driver is by way of rail-ladders installed inside the mast of the tower crane. There may be a vertical ladder with a protective safety cage, or “crinoline,” or multiple oblique ladders connecting resting landings disposed at regular vertical intervals.
The main drawbacks of this traditional mode of access are the necessity for a large physical effort by the crane driver to ascend the mast to the cabin and the long time necessary to ascend to the cabin and conversely to descend to the ground, which reduces the productivity of the site on which the tower crane is used.
To avoid these drawbacks motorized height access solutions have already been proposed. These solutions typically consist of providing a “lift” or elevator that is installed and moves either on the outside of the mast or on the inside of the mast.
Motorized solutions on the outside of the mast still have many drawbacks:                The access device, otherwise known as the lift, must be installed on the mast after assembling the tower crane. Having to wait until the tower crane is assembled to then install the lift increases the time to assemble the combination of the tower crane and its lift.        The presence of a lift on the outside of the mast is incompatible with the use of a telescoping cage, which also is situated on the outside of the mast.        The presence of an outside lift increases the cross-section of the tower crane exposed to winds, which can limit the working height of the tower crane.        The presence of an outside lift also entails the risk of the lift colliding with bulky loads suspended from the hook of the tower crane.        In the case of a lift moved by a rack and pinion mechanism, such a mechanism is incompatible with any movement or swaying of the mast, which is overcome by providing an intermediate mast on which the lift will move.        In the case of a lift drawn by a cable, an upper fixed point outside the mast must be provided.        There is often a challenge for persons to move between the lift positioned on the outside of the mast and the inside of the mast.        
The patent documents CH665825 and EP 0 175 052, as well as the patent document WO 2005/087645, provide examples of motorized solutions on the outside of the mast in which the lift cabin on the outside of the mast is accessed by a working platform.
Accordingly, motorized solutions inside the mast of the tower crane appear preferable for avoiding the drawbacks indicated above. Such solutions are described in the patent documents WO 92/18412 and FR 2 936 236.
In the case of the document WO 92/18412, a simple lift cabin is mounted and moves inside a safety cage that is installed inside the mast. The lift cabin is guided along the ladder and is moved by a rack and pinion mechanism, the rack preferably being carried by the ladder.
In the case of the document FR 2 936 236, a lift cabin is mounted to move on a vertical guide rail inside the mast, the guide rail being equipped with a rack for moving the lift cabin. This lift cabin is surmounted by a working platform surrounded by a guard rail. The lift cabin and its working platform cannot be separated. The mast of the tower crane is formed by the superposition of a certain number of mast elements and the combination consisting of the lift cabin and the working platform can be located entirely inside the bottom mast element, where it is installed for the transport of the dismantled tower crane. A trapdoor through which a vertical ladder passes provides access between the inside of the lift cabin and the working platform surrounding the cabin.
In such an embodiment, however, the permanent fastening together of the working platform and the lift cabin has drawbacks including, in particular:                The lift cabin and the platform surrounding this cabin makes it obligatory to vertically move a large mass and a large volume resulting from each time the crane driver ascends or descends.        Most importantly, the solution from the document FR 2 936 236 creates an undesirable situation because the platform remains with the lift. When the lift cabin is in the bottom position at the base of the tower crane or moving along the mast the platform moves with the lift. This, however, means there is not a platform adjacent the driving cab, which leaves a large gap at the level of the driving cab of the tower crane, regardless of whether or not anyone is working in this area.        When the tower crane is in service, the platform and the lift cabin are situated in the upper part of the mast, with the result that the cross-section of the tower crane exposed to winds is increased because of the presence of the lift cabin at a great height above the ground.        If the working platform must be stopped at intermediate heights for particular work to be carried out or during operations of mounting or dismantling the tower crane, the lift cabin is necessarily stopped at the same level as the platform and is no longer usable for operators to ascend or descend.        